1. Field of the Invention
The present invention relates to a hydraulic jack with a system for checking the position of the piston.
2. Description of the Prior Art
In many applications of jack-operated hydraulic control systems, it is necessary to determine the position of the element which is actuated by the jack. To this end, it is a customary practice to make use of so-called end-of-travel contacts or passing contacts which are influenced by the element itself or by an associated element such as, for example, the emergent rod of the piston.
However, for certain specific applications such as, for example, hydraulic control of high-voltage electric circuit-breakers, it is highly inconvenient and often impossible (on account of the voltage applied to certain moving parts) to install end-of-travel detectors or the like in immediate proximity to elements which are actuated by the jack.
It is for this reason that, in order to avoid mechanical connections of substantial length and low reliability, provision is made in electric circuit-breaker control systems for a so-called "image jack" or in other words a small auxiliary control jack which is supplied or connected to drain in the same manner as the main jack. Thus the auxiliary jack is intended to reproduce the operations of the main jack and consequently to reproduce the displacements of the element actuated by said jack, namely the moving contact of the circuit-breaker.
Aside from its principal function which consists in indicating the open or closed position of the circuit-breaker, the "image jack" also has the function of controlling certain automatic safety operations of the circuit-breaker. This jack must therefore offer highly reliable operation, with the result that it is difficult and therefore costly to manufacture with a view to guarding against any danger of a false indication. A further disadvantage of such jack is that it entails the need for an additional hydraulic circuit, which may be a source of leakages of hydraulic fluid.
Furthermore, in modern circuit-breaker control systems, it is a desirable objective to check the real response and operating times of the moving contact of the circuit-breaker under service conditions. It is known that the operating times are of very short duration, namely of the order of a few hundredths of a second. These time intervals are checked at the moment of reception of equipment but it is no longer possible to determine whether initial performances in fact continue to be maintained once this equipment has been put in service. It would also prove highly advantageous to make sure that these operating times are wholly constant and reproducible, in particular in the case of circuit-breakers having a number of poles in series in which operations must be absolutely simultaneous. Finally, electrical engineers take into account the time of outward displacement of the moving contact with respect to the stationary contact (this time interval being clearly only a fraction of the total time of displacement of the moving contact) and also need to determine the curves of velocities of the moving contact.
It will be readily apparent that, since the inertia of the "image jack" is not at all the same as that of the main jack and the moving parts which it actuates, said "image jack" is not capable of giving any useful indication in regard to the different operating characteristics under service conditions.
The object of the present invention is to overcome the disadvantages and shortcomings of the control systems such as "image jacks" in particular which have been in use up to the present time.
The present invention permits the construction of a control system which detects the position of the piston itself during its travel within the jack, position detection being possible not only in the two end-of-travel positions but also in a plurality of intermediate positions.
Up to the present time, it has not been possible to house piston-position sensors within a jack cylinder in immediate proximity to the displacement of the piston. In point of fact, conventional jacks designed for circuit-breaker control systems operate at a very high pressure of the order of 300 to 400 bar, which calls for perfect pressure-tightness of the piston within the cylinder. The piston is fitted with a packing seal of a highly elaborate type which consists in the majority of instances of a so-called "spring-loaded packing" and the internal surface of the cylinder is perfectly ground and lapped so as to guard against either wear or damage of the packing. It is clearly not possible to place one or a number of piston proximity detectors within the thickness of the cylinder wall since the resultant irregularities in the surface of the cylinder would have the effect of destroying the packing.
The present inventor has recently found, however, that it was possible to construct differential jacks, especially for the control of circuit-breakers, in which the piston is not provided with any packing seal or ring and in which the piston actuates or is adapted to carry a sealing valve for closing the main chamber of the jack at the end of travel of the piston. In consequence, the leakage flow which exists around the piston by reason of the fact that this latter is no longer provided with any packing seal is prevented from penetrating into the volume of the cylinder located beneath the piston when this latter has reached the end of its travel.
A differential jack of this type in which the piston is not fitted with any packing seal has been described in U.S. patent application Ser. No. 168,148 filed Mar. 15, 1988 in the name of the same inventor.
The present invention applies to a differential hydraulic jack of this type.